1. Field of the Invention
The present invention relates to data communication systems and, more particularly, to data communication systems transmitting and receiving signals in the presence of unintentional background noise or intentional interference.
2. Description of the Related Art
The ability to detect and acquire low level communications, navigation, or radar signals in the presence of unintentional background noise or intentional interference (jamming) has been and continues to be a critical requirement for numerous military and civilian systems.
Traditional solutions include increasing the transmitted signal power, sometimes adaptively, to allow operation with various levels of interference. One such application is for the next generation of the Global Positioning System (GPS). The military community requires additional anti-jam (A/J) capability for the GPS military signal in theaters or areas of responsibility (AORs) during times of conflict. One proposal that has been submitted to achieve this capability is to configure the GPS satellites with large, steerable dish antennas that can direct high power spot beams into an AOR.
Another solution, for synchronous communications systems, is to increase the signal power in short bursts with duty cycles that are of the order of the reciprocal of the power increase. With this scheme, the average power is not increased and, since the system is synchronous, the receiver can anticipate when the bursts, or pulses, will occur. If the receiver is only trying to acquire the signal during the occurrence of a pulse (or pulses), then the resistance to interference is improved by the amount of the instantaneous power increase.
However, the brute force technique of raising the average power to counteract interference or jamming generally is expensive and is limited by the power resources available at the transmitter. The proposal to increase power in a selected geographical area via spot beams for the next generation of GPS is extremely expensive, in terms of cost, operational complexity, and size, weight and power of the required satellite equipment.
The pulsed power technique discussed above is an elegant solution, but it requires synchronous operation. The receiver must have its internal clock synchronized with the transmitter's clock, in order to be able to anticipate the occurrence of the high power signal pulses. If the receiver cannot predict when the pulses will occur, then the receiver will continuously attempt to acquire the signal. Accordingly, no advantage will be realized from the pulsing technique, because the total signal energy in the receiver's acquisition window will be the same as without pulsing. Also, if the power is pulsed at a regular rate, a jammer can pulse the jamming power at nearly the same rate so that periodically the jamming pulses will synchronize with the signal pulses at the receiver, giving an advantage to the jammer to at least periodically block the signal.
Accordingly, there is a need for a system that increases the signal power in short bursts but that does not require synchronous operation.